Creating more durable colors
Researchers from the Adolphe Merkle Institute’s Soft Matter Physics group are developing new pigments that are potentially more environmentally friendly and durable thanks to structural color.
Photonic pigments, also known as structural colors, are pigments that do not rely on chemical dyes to produce color. Instead, they are made up of nanostructures that can manipulate light interactions with matter, causing it to reflect and refract in patterns to produce a particular color. The exact hue that is produced depends on the size, shape, and arrangement of the structures within the pigment. For example, a butterfly's wing may appear blue or green due to how light interacts with the microscopic structures on its surface. Unlike chemical pigments that can fade over time, photonic pigments aim to be highly durable and resistant to fading, making them potentially useful in applications such as cosmetics, paints, textiles, or even security features on banknotes. They also offer pure and brilliant coloration free from chemical- or photo-bleaching, potentially reducing their environmental impact.
The creation of color through photonic morphologies has focused chiefly on confining the self-assembly of colloidal particles or liquid crystals in specific geometries. However, synthesizing large quantities of photonic pigments based on these amorphous arrays results in a lack of control over how light is scattered and highlights the difficulty in producing pigments with distinct colors across the entire visible spectrum.
The AMI Soft Matter Physics group has investigated, instead, as part of a European-funded project, another promising approach using 3D confined self-assembly of block copolymers - chains of different monomers linked together - in emulsion droplets to create these pigments. Previous attempts have required complex technical solutions, and developing robust fabrication processes has proven arduous, limiting its technical exploitation.
The new process involves using a block copolymer and two swelling additives in a solution, which is then emulsified using a vortex to form droplets. As solvent diffuses from the droplets, they form spheres with a lamellar, or onion-layered structure. The thickness of the lamellae can be controlled by differential swelling with the two additives, resulting in the ability to tune the color of the spheres across the entire visible spectrum.
“We were able to identify the necessary parameters that allow for the scalable, robust manufacture of these photonic pigments, which are also tunable across the entire visible range by only varying the compositions of the particles,” explains lead researcher Andrea Dodero. “And importantly, all these particles can be made from the same block copolymer, and no additional synthesis is required to create different colors.”
To achieve this, though, some important factors need to be considered. Of the two swelling agents required, one should strongly interact with one of the block copolymer constitutive chains. The additives should also be chosen to increase the difference in domain dielectric contrast, and the spheres need to form slowly enough to develop the layered structure that creates the colors.
The next steps for the researchers according to Dodero include combining the different photonic pigments with broadband absorbers enabling the reduction of scattering effects and enhancing the overall color appearance, and creating structurally colored photonic coatings and paints for real-life applications.
References:
Dodero, A.; Djeghdi, K.; Bauernfeind, V.; Airoldi, M.; Wilts, B. D.; Weder, C.; Steiner, U.; Gunkel, I. Robust Full-Spectral Color Tuning of Photonic Colloids. Small 2023, 19 (6), 2205438.
Steiner, U.; Dodero, A. Block Copolymer-based Photonic Pigments: Towards Structural Non-iridescent Brilliant Coloration. CHIMIA 2022, 76, 829.